MIC AND AUDIO AMPLIFIER CIRCUIT

Today we are going to discuss about a Low cost, compact
Audio Pre-Amplifier with Loud speaker. By using this device you can hear your
voice loudly with a better quality. The circuit provides an output power of
about 0.5 watts by just 5V supply. It can be used as a part of intercoms,
walkie-talkies, pocket radio receivers etc… all the components used in this
project are readily available in the market at low cost.

COMPONENTS REQUIRED

IC 1 – BEL1895

T1 & T2 – BC148
NPN

Loud Speaker 4Ω 1W

Condenser Mic

Battery 6v

CIRCUIT DIAGRAM

WORKING PRINCIPLE

For better understanding we can dividethe circuitinto two sections

1) High gain Mic
Pre-Amplifier

2) A.F Power
Amplifier

High Gain Mic
Pre-Amplifier

It consists of a condenser mic, a variable resistor and two
NPN transistors (BC148) with some passive components such as Resistors and
Capacitors.

Condenser Mic

It is basically a transducer, used to convertaudio(speech) signals to electrical signals. The
commonly used condenser mic’s are cylindrical in shape in which its top most
part is enclosed by a sensitive diaphragm. The diaphragm vibrates with respect
to our voice, hence the carbon granules filled inside it. This offers a
variation in output resistance which in turn varies the voltage directly. This minute
variation in electric voltage with respect to our voice is amplified in the
further stages.

Variable Resistor

It isa 10kΩ
variable resistor, used to control the gain of audio pre-amplifier by adjusting
this we can increase or decrease the output gain.

NPN Transistors T1
& T2

The circuit uses two stage RC coupled amplifier to increase
the gain of signal coming from condenser mic. The transistors are biased by a
voltage divider network to provide proper stabilization.

A.F Power
Amplifier

Audio frequency power amplifier section consisting of IC BEL
1895, itis a monolithic audio power
amplifier designed for AM radio applications. It delivers 1Watt output power at
6V supply voltage. BEL1895 ic exhibits low distortion and noise with an
operating range of 3-9V. The output is taken from pin1 which drives the loud
speaker.